Fluorescent lamp assemblies are used in both retrofit and original installation configurations. Compact fluorescent lamp fixture adapters are most useful in retrofitting existing Edison-type ceiling lighting fixtures so those previously installed fixtures can use the energy efficient lighting sources currently available. Particularly, in retrofitted fixtures, constrictive thermal environments create conditions which often cause fluorescent lamps to operate at less than their maximum efficiency.
The operating efficiency and lumen output of compact fluorescent lamps is very sensitive to lamp wall temperatures. Elevated lamp wall temperatures occur as the geometry of the reflector and the lens traps air within the lamp compartment of typical compact fluorescent retrofits. This in turn inhibits desirable cooling of the lamp wall.
Compact fluorescent lamps operate at a maximum efficiency when the mercury pressure inside the fluorescent tube is at its optimum value (e.g. 5 to 6.times.10.sup.-3 mmHg). Moreover, when the mercury vapor pressure of the fluorescent tube is kept low and the current flowing through the fluorescent lamp is prevented from increasing, the burning of the reactance ballast due to generated heat may be advantageously prevented.
Many residential and commercial retrofit applications of the compact fluorescent lamp use a lensed reflector geometry which results in significant losses in performance due to excess mercury vapor. This constricted configuration is necessary to focus and direct the light output of the lamp.
Convection cooling is most effective when there is an absence of a lens or reflector. In such a case, the compact fluorescent lamp operates near maximum performance. This occurs because the convection cooling allows excess mercury vapor to condense and coalesce at the coolest point on the lamp's surface thereby maintaining an advantageous range of mercury vapor density. This coolspot is located at the lamp tip and is termed the minimum lamp wall temperature (MLWT). The optimum location of the coolspot is at the lamp tip. If the MLWT occurs at this point, the lamp is most efficient.
With the addition of a lens and reflector, however, the compact fluorescent lamp wall temperature rises, causing the system to experience loss of light output and efficacy dye to the lack of an efficient coolspot at the highest density of mercury vapor. The excess mercury vapor results in increased self absorption losses and therefore leads to lower lumen output. Recent experimental work has shown that light output for conventional compact fluorescent lamps can decrease by as much as 30% when operated with a lens and reflector inside a standard fixture.
The benefits of convection cooling are described in U.S. Pat. Nos. 4,375,607 and 4,270,071, the former of which describes a configuration having a very large aperture at the distal end of the light shield or light diffusing member as well as small apertures adjacent the ballast, providing a convection cooling effect of both the lamp and the ballast. The latter references describes a less radical configuration where small apertures are cut into the distal end of the transparent light transmitting housing as well as having small apertures adjacent the ballast.
In each of the aforedescribed approaches, convection cooling is accomplished in varying degrees. However, optical effects of the configurations of apertures on the transmitted light are not considered. Moreover, neither approach provides for the ability to use a reflector to direct light toward the housing's distal end. While the first approach eliminates the possibility that insects will be trapped by the distal end of the lamp housing, the second approach does not. These and other advantages are provided by the above discussed prior art. In general, the major barrier to the inclusion of venting geometries on lensed fixtures has been the potential for dirt and insect accumulation. By including vents means that dirt and insects can enter the can easily and then accumulate to reduce lumen output of the fixture through dirt depreciation.